The class of compounds commonly referred to as pyrethrins is generally understood to include the four major active ingredients of pyrethrum viz. Pyrethrin I, Cinerin I, Pyrethrin II and Cinerin II. Pyrethrum is the commercial extract of the herbaceous perrennial Chrysanthemum cinerariaefolium and is an important source of natural insecticides. The pyrethrins which, as noted above, comprise the major insecticidal constituents of pyrethrum and are especially useful in insecticides by virtue of their increased insecticidal potency when used with known synergists, e.g., piperonyl butoxide, their low mammalian toxicity, rapid knockdown or paralytic properties, and the absence of induced insect resistance from exposure to sublethal doses.
Owing to their importance, the preparation of pyrethrins has been the subject of much investigation. There are, however, no commercially acceptable syntheses available for the preparation of the pyrethrins and the entire commercial production of pyrethrins is accomplished by extraction from pyrethrum flowers. Such commercial extraction procedures involve treatment with organic solvents which invariably results in the extraction of substantial amounts of inactive and undesirable impurities necessitating costly and cumbersome purifications in order to obtain insecticidally useful pyrethrins. The novel process of this invention provides a method for the synthesis of several of the pyrethrins and pyrethrin analogs via easily accessible starting materials.